1. Field of the Invention
The present invention relates generally to a control system for an automatic transmission and more specifically to a pressure control arrangement which produces a constant pressure signal which signal is supplied to various valves in the form of a source pressure.
2. Description of the Prior Art
U.S. Pat. No. 4,730,521 issued on March 15, 1988 in the name of Hayasaki et al, discloses a transmission control arrangement which includes a shift shock suppression arrangement of the nature shown in FIG. 1. In this arrangement so called pilot pressure PP is produced by a pilot pressure valve 1 by using line pressure as a source pressure and modifying the same in order to form a pressure having a single constant level.
As shown in FIG. 1 this pilot valve 1 comprises a spring 2, and a spool 4. The spring 2 is arranged to bias the spool 4 in the bore 5 it is reciprocatively disposed in a direction wherein the volume of a feedback chamber 6 tends to be minimized.
The spool 4 is formed with two lands 4a, 4b while the bore is formed with five ports 5a-5e. When the spool 4 assumes the position shown by the upper sectional half, drain port 5d is completely closed and supply port 5b opened. Under these conditions line pressure PL (source pressure) is permitted to pass from conduit 8 via port 5b to port 5c and into pilot pressure conduit 10. As shown, conduit 10 is fluidly communicated with the feedback chamber 6 by way of a branch conduit 10a in which an orifice 12 is disposed. The pressure which develops in the feedback chamber 6 produces a bias which acts against the bias of the spring 2 and the spool 4 is moved back toward the position shown in the lower sectional half wherein supply port 5b tends to close and drain port 5d begins to open. Depending on the strength of the spring 2, the pilot pressure PP is modulated to a constant level due to an equilibrium being established between the opposed forces.
As shown, conduit 10 is arranged to communicate with a number of valves including a solenoid valve 14. This valve 14 is arranged to produce a control pressure PC by varying the duty cycle of the signal applied to the solenoid and therefore varying the amount of pressure which is arranged to be bled off downstream of an orifice 16. In this instance this pressure is arranged to vary with with load (i.e. define throttle pressure or a pressure signal which varies in accordance therewith).
This control pressure PC is supplied to a pressure modifier valve 18 along with the pilot pressure PP and is used to modify the level of the pilot pressure in a manner which generates a line pressure control signal which is supplied to the line pressure regulator valve 20.
However, in this arrangement an oil strainer or filter 22 which is designed to remove various small particles from the hydraulic fluid, is arranged in conduit 10 so as to be located in series in between the pilot valve 1 and the valves to which the pilot pressure PP supplied (e.g. solenoid valve 14) and thus prevent jamming and the like improper operation the same. This filter 22 is relatively small in size and therefore tends to increase the flow resistance of the circuit in which it is disposed. This has been discovered to lead to the drawback that the level the pilot pressure PP downstream of the filter 22 undergoes a reduction. This effect increases as the amount of material which is filtered out accumulates and further increases the resistance of flow. Due to the lowering of the pilot pressure PP, as the duty cycle of the signal applied to the solenoid valve 14 is derived assuming that the pilot pressure will assume a predetermined level, the lowering of the pilot pressure PP leads to the situation wherein the throttle pressure (PC) is also lowered. This effects the level of the pressure signal which is fed to the line pressure regulator valve and thus has a detrimental effect on the control of this pressure also.
For a more complete disclosure of the above arrangement refernence may be had to the above mentioned U.S. Pat. No. 4,730,521 the disclosure of which is hereby incorporated by reference thereto.